In a vehicle having a regenerative brake function, a drive motor doubles in function as a motor for traveling the vehicle and a generator for regeneration. When the vehicle is in deceleration, the drive motor converts deceleration energy into electric power to thereby generate a regenerative braking force. More specifically, recovery of electricity begins from the moment the driver releases its foot from an accelerator pedal. When a brake pedal is stepped on or operated, deceleration of the vehicle is enhanced and a greater amount of electricity is created and charged into an on-vehicle battery. In order to realize a regenerative brake function, the vehicle is equipped with an electric servo brake system which cooperates with a drive electronic control unit (drive ECU) in performing regenerative control.
In an electronic control unit having a motor load, such as the electric servo brake system having a regenerative brake function, an electrolytic capacitor for removing ripples is provided, with one end grounded, in a load power supply line connected to a positive terminal of the on-vehicle battery. Also connected to the load power supply line is a failsafe relay which interrupts electric power from the on-vehicle battery in case of emergency. Upon operation of the failsafe relay, the motor load is separated from the on-vehicle battery with the result that a load circuit is protected from a fault such as an overcurrent fault.
The electronic control unit may encounter a ground fault which occurs at an arbitrary part or position of the load power supply line. The load power supply line is used herein to refer to a power supply line which extends from a downstream end of the failsafe relay to the motor load. When a ground fault occurs at an arbitrary part or position of the load power supply line, a constant resistance component is set between the arbitrary position of the load power supply line and the grounded vehicle body. Conventionally, a current flowing in such ground-fault position is detected by monitoring with a special current detecting device such as a shunt resistance or a Hall element, as disclosed, for example, in Japanese Patent Application Laid-open Publication (JP-A) No. 2000-108887. According to another known ground-fault detection method, a voltage drop at the ground-fault position is monitored.
However, the conventional ground-fault detection method using a current detecting device requires a dedicated circuit having a current detecting function and, hence, is disadvantageous in terms of the cost and component packaging density. Furthermore, the ground-fault detection method relying on voltage monitoring at the ground-fault position involves a reduction in detection performance when a voltage drop resulting from a ground-fault does not reach a prescribed detection threshold value.
It is therefore an object of the present invention to provide an electronic control unit which is capable of improving detection performance against a ground-fault having a resistance component without requiring a dedicated current detecting circuit.